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Just about the last thing anyone expected to find on that soggy trek up Mimico Creek was a sea-water crustacean known as the blue crab, notorious for the vice-like grip of its claws.

But the hikers, who were carrying out an erosion survey on behalf of the Toronto and Region Conservation Authority, happened upon a half-dozen of the beasts, mostly immersed in the slow green waters of the Mimico, where it drifts and eddies beneath the four-lane Bloor Street overpass, just west of Our Lady of Sorrows church.

What’s more, the creatures were alive, albeit a long, long way from their home and native land.

Make that home and native sea, for the blue crab is a salt-water denizen, whose natural habitat extends from the briny waters off Nova Scotia all the way south to the faraway coast of Uruguay, with the greatest concentrations to be found in Chesapeake Bay and along the salty shores of North Carolina and Louisiana.

“It’s got a pretty extensive natural range,” says biologist Dayna Laxton, who is writing an interagency report on the find. “But why were they in a creek in Toronto?”

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That is the question, and it lies at the heart of a baffling urban mystery, one that might well be dubbed “The Curious Case of the Errant Arthropods.”

Ever since the creatures were found last July, experts at the federal, provincial and local levels have been earnestly trying to solve a series of riddles. When, for example, did the crabs get here — “here” being a location very near the intersection of Bloor Street and Aberfoyle Crescent? How did they get here? And what does their presence imply for the environmental challenges facing this city?

In other words, are we being invaded by alien crustaceans?

Actually, this last question has already been answered: probably not.

The experts have managed to rule out any realistic chance that the infamous blue crabs of Mimico Creek somehow propelled themselves independently all the way from the distant Atlantic swells to a point that’s approximately halfway between the Royal York and Islington subway stations on the Bloor-Danforth line.

“They’re a good swimming crab,” says Laxton, “but they’re not that good.”

No, in order for the crabs to get to Toronto, someone had to bring them.

But that still leaves plenty of room for confusion and doubt. Who brought them? By what means? Why?

And the biggest question of all: how did they survive?

Revered by seafood lovers for the delectable quality of their meat (following roughly 30 minutes of steaming, of course), blue crabs are by no means rare, at least not among the varied aquatic fauna to be found in the Atlantic waters off the east coast of the Americas.

But there’s the rub.

Even at its closest point, the east coast of the Americas is a long way from Toronto’s west end. What’s more, the Atlantic Ocean is highly saline, whereas Mimico Creek is a freshwater stream — right?

Well, so you might have thought, but the times they are a-changin’ and not necessarily for the better.

“We use road salt, and these streams are so impacted,” says Laxton. “This is a water-quality parameter that should get more attention.”

Is it possible that Mimico Creek has become so salty that it’s able to support sea-dwelling creatures such as the blue crab? And not just in winter or spring, but in summer, too? After all, the crabs were discovered in late July.

“One would assume that a majority (of the salt) would have flushed out by then, but I wouldn’t be surprised if there’s some residual,” says Laxton. “It’s a highly urbanized creek.”

Provincial officials promptly ran tests to determine the salinity of the stream, using a device known as a salt pen. They found chloride readings that were significantly higher than expected.

But, late in August, experts from the federal Fisheries and Oceans department did their own tests, using more sophisticated equipment.

“They found saline levels that were not nearly high enough to sustain these crabs,” says Laxton.

Those findings raise two possibilities — either the level of salt in the creek was never high enough to support the crabs, or not for long, or the level plummeted between July and August.

Nobody seems to know how long the crabs were able to survive in the urban wilds of Bloor Street West. Live specimens were first found on July 15 and again on July 21. Following July 27, only dead crabs were found.

Whether living or dead, all of the Mimico Creek blue crabs proved to be mature females.

“Was the stream really, truly high enough in salinity to support them?” wonders Laxton. “Or were they just able to survive for a time — not happy but able to do it?”

It turns out that blue crabs don’t absolutely require salt water to survive, or not all the time. In fact, they actually seek out inshore brackish water for reproduction — although there is no evidence the creatures were here on a sort of group honeymoon.

(For that, they would have gone to Niagara Falls, no?)

“The long and short of it is, we didn’t find any larval crabs,” says Scott Jarvie, manager of watershed monitoring and reporting at the conservation authority.

During the course of their research, the Canadian experts also searched the Internet for clues, and they turned up a series of photographs posted by a local woman who had stumbled upon some blue crabs at Mimico Beach in April of last year.

Could these two finds possibly be related?

Laxton thinks not.

For one thing, Mimico Beach is at least six kilometres from Our Lady of Sorrows, as the crab swims, and the route is interrupted by at least one formidable dam — formidable, that is, for a crab.

“We came to the conclusion these were two separate events,” Laxton says.

The truth is, blue crabs are far from unknown in Toronto. It’s just that, normally, they arrive in containers aboard airplanes and are then transported in trucks from Pearson International to any of several seafood emporia in the city.

“You can buy them live, fresh,” Jarvie says.

Laxton wonders whether the crustaceans fell from a truck as it travelled along Bloor Street over the Mimico Creek. Or maybe someone deliberately tossed the crabs into the stream — who knows why? Finally, some not very clever individual might have been trying to stock the creek with edible arthropods, perhaps in hopes of reaping a profit.

But nobody knows.

There has yet to be a recurrence of the still-confounding discovery — and so the mystery of Mimico Creek scuttles on.

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